Data storage disk drives are used for high volume storage of data capable of being written or read by means of a special head arrangement while the disks are rotated on a spindle mechanism at a predetermined speed. The data in a suitable form resides in circular tracks disposed concentrically on the disk surface. To increase the amount of data to be stored on a disk drive device such as a hard magnetic disk drive, several hard magnetic disks are rigidly secured to a rotational spindle hub together in a suitable disk stack configuration.
To ensure reliable data transfer and storage operation of the disk drive, the hard disk stack must be accurately positioned on the spindle hub relative to the spindle's axis of rotation. Also, relative radial displacement between the read/write head and a desired disk track must be tightly controlled.
In practice, the spindle hub and the attached disks are rotated by means of a direct current brushless motor which is enclosed inside the hub so as to reduce a space requirement for the spindle driving mechanism.
To complete the motor flux return path and shield the disk surfaces from stray magnetic interference, a soft iron or steel sleeve of generally cylindrical shape and having desirable magnetic properties is usually inserted inside the spindle hub. The sleeve is also used to carry the permanent magnet part of the motor which is generally bonded to the inside wall of the sleeve. The hub and disks are generally made of the same material, e.g. an aluminum alloy, or of materials with the same coefficient of thermal expansion to ensure that thermal expansion of the hub and the disks, inherent in normal disk drive operation, does not result in undue distortion of the hub and dislocation of the disks from their desired spatial orientation.
In the past, fastening the sleeve to the spindle hub was accomplished by adhesively bonding the sleeve's outside wall to the inside wall of a cavity within the hub. The need to insert the motor inside the cavity resulted in the walls of the spindle hub in the vicinity of the sleeve attachment being substantially thinner than the remainder of the hub's construction. As a consequence, the spindle hub/sleeve interface was susceptible to increased stresses and deformation incident upon the disparity in the material and thermal expansion properties of the sleeve and the spindle hub. Under typical operating conditions of a disk drive, unequal thermal expansion of the sleeve and the hub would result in significant distortion of the hub's cavity wall displacing the disks adjacent to the wall relative to the read/write heads and the remaining disks. The hub shoulder, which is used to support the bottom disk, may undergo similar displacement. The resultant disk misalignment on the order of as little as several microinches is highly undesirable because of its deleterious effect on the spindle balance and disk track eccentricity. The latter problem is a cause of the so-called disk runout and track offset which adversely affects the accuracy and speed of positioning the read/write heads relative to the desired disk track. This in turn results in longer data access times and potential data transfer errors which degrade reliable operation of the disk drive.
To alleviate the problem of the hub's thermal distortion it was known to reduce the length of the adhesive bond between the sleeve and the hub. However, this measure generally results in only a partial improvement.
An alternative approach tried in the past is to construct the spindle hub and the sleeve of the same ferromagnetic material. With the two parts expanding and contracting at the same rate, the distortion of the hub is avoided. However, since the disks are invariably made of a non-ferromagnetic substrate, such as as an aluminum alloy, there is a thermally induced distortion of the disks at the spindle hub/disk junction.